TECHNICAL
Chamber correction EQ
Physical Design
The physical design of a Tetrahedral Test Chamber, is based upon the triangle which is amongst the strongest known structures; also differs from test boxes. By minimising parallel surfaces, these in turn reduce modal effects that seriously interfere with measurement chambers.
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The measurement microphone is fitted inside the chamber at a known and easily set distance. The loudspeaker to be tested is placed so that it's sound is directed into the chamber - towards the microphone, not out of it like a normal loudspeaker enclosure.
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Individual sub baffles are then used to adapt individual loudspeaker drive units to the test chambers allowing rapid and very accurate physical location which equates to more stable measurements.
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We have experience integrating TTC's into production environments.
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How Accurate and How Consistent is a TTC?
The top graph shows the measurements of a single SEAS loudspeaker drive unit #7 measured fifty times with the chamber equalisation applied to a TTC900. The one-third octave curve is the error level less than 0.2dB at 95% confidence level, this from 20Hz to 10kHz, even at the loudspeaker drivers resonance! Up to 20kHz where the SPL drops off.
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The lower graph shows the results of a seriers of these loudspeaker drive units #2 to #7 but measured sixty eight times. This shows greater variation as would be expected from different loudspeakers. Note: The minimum variance shown in the mid band is 0.25dB, is in agreement with the first graph, so these are genuine loudspeaker response variations.
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These graphs demonstrates the control and stability of the measurements that a tetrahedral test chamber can provide, wherever they are used. R&D, QC, production, goods in or repair and refurbishment all can benefit equally.
Consistently Stable Loudspeaker Measurements using a Tetrahedral Enclosure, AES 135th Convention, 2013 October 17-20 New York, USA
Comparative Results between Loudspeaker Measurements Using a Tetrahedral Enclosure and other Methods, AES 136th Convention, 2014 April 26-29 Berlin
TTC350 on the cover and in Voice Coil Magazine, November 2014
What is a Tetrahedral Test Chamber? - 2015
Hill Acoustics Tetrahedral Test Chamber Whitepaper - 2015
Loudspeaker Modelling and Design A Practical Introduction - Geoff Hill, published by Routledge 2019
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Publications & Articles
How can you create an accurate chamber correction curve and how can you KNOW it is accurate?
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This is probably the number one question for a new user who is used to either anechoic or free field conditions.
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Most loudspeakers are described as "Pistonic" which means at low frequencies below the cone break-up region the whole diaphragm moves as a single mass or entity, so the sound pressure created at the front of the diapragm is and must be the same as at the back of the diaphragm.
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We can use this to accurately calculate the difference between the internal pressure response and the near field external response and this allows us to create an accurate chamber correction curve.
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The next step is to ensure that the chamber interfere's with the measurements as little as possible and this is covered by the physical design of the TTC.
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